Method for preservation of fresh fish or sea-food

ABSTRACT

A gas mixture for preserving fish and sea-food products, comprising carbon dioxide, oxygen and argon with partial pressures in the following range: 
     
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            CO 2      50%-68%                                               
       O 2        5%-20%                                               
       Ar           27%-45%                                               
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     This mixture slows down enzymatic reactions and microbiological development at the surface and inside the fish or sea-food product.

This application is a division of application Ser. No. 07/203,415, filedJune 7, 1988 now U.S. Pat. No. 4,946,326.

BACKGROUND OF THE INVENTION

The usual industrial practice of catching fish or harvesting sea-foodproduce is to bring them on board of the fishing boat where processingof the catch is operated or not. Most of the time, fishing boats remainat sea for 5 days or more: therefore, it becomes necessary to processthe catch as soon as possible and prevent it from turning bad. This isespecially true for the catch obtaining during the early days at sea. Acommon practice is to store the catch in a refrigerated space mixed withan equivalent amount of crushed ice. When brought back to the port, thecatch is either sold as fresh or more likely processed in various wayssuch as canning, smoking or freezing.

As soon as the catch is taken out of its living environment (sea water),degradation processes start. Chemical and bacteriological reactions arethe major sources of fresh fish and sea-food degradations. Bacterialcontamination has many origins. Part of it comes from ambient air, buthandling, cleaning and storing operations are the major responsiblefactors for contamination of the product. When living in its naturalenvironment, edible parts of fish or sea-food have an extremely lowlevel of contamination (e.g., below 10 germs/g). However, it is notunusual to observe that level rising to 10⁴ or more germs/g after theproduct has been through every processing step. The higher the initiallevel of contamination, the faster the deterioration of the product willbe and ultimately, part of the caught stock has to be destroyed becauseof its total loss of market value.

For a long period of time, it has been suggested to extend theshelf-life of fresh sea-food and fish by packaging these products undermodified atmosphere in order to limit the microbial development on. It,has been suggested to use enriched or even pure CO₂ atmospheres. It isknown from U.S. Pat. Nos. 3,851,080 and 3,930,040 and French Patent81/20784 to use a binary or ternary mixture of N₂ and CO₂, O₂ and CO₂ orO₂, N₂ and CO₂, with partial pressures in the range of 50% N₂ and 50%CO₂ or 30% N₂ and 70% CO₂ or 20% O₂ and 80% CO₂. These compositions areappropriate and efficient for the extension of the shelf life of theseproducts. CO₂ has an inhibiting effect on a wide number of microbialgerms and a good solubility within fats and moist products. Therefore,it has become a basic requirement in gas packaging techniques to use CO₂with a partial pressure as high as possible inside of the container.However, for moist products such as sea-food and fish, partial pressuresof CO₂ above 70% have the tendency to acidify the product. Somebleaching effect of the product due to elevated CO₂ partial pressuresmay as well be observed. On the other hand, it has been shown that CO₂does not have an inhibition effect on the growth of strictly anaerobicgerms such as Clostridium.

Fresh fish or sea-food can be naturally contaminated with spores ofClostridium botulinum. In strict anaerobic conditions, these spores aresusceptible to germinate and produce a toxic protein which isexcessively dangerous for mankind. Risk of development is particularlyhigh when the storage temperature of the package exceeds 4° C. Thistemperature has always been a difficult parameter to control all alongthe distribution chain of the product, and it is absolutely necessary toprevent the development of this germ in case of an accidentaltemperature rise. Introducing oxygen inside of the container or packageis a recognized and accepted method to avoid Clostridium developments,and therefore an O₂ partial pressure of at least 5% is usuallyrecommended for the final formulation of the gas mix. Nitrogen is usedas a complimentary gas providing space filling or dilution but it is noteffective for controlling the microbial development.

Using the various gas combinations available today for sea-food or fishpackaging extends its shelf-life from 2 days (air) to 8 days (gas).Exceptionally, products featuring a very low level of initialcontamination and low fat content can have their shelf-life extended to12-15 days. However, considering the length of the fishing campaign, thetransportation time required for the distribution of the product and thestorage time on distributor's shelves, a shelf-life of 15 days is aminimum requirement for an efficient marketing of any fresh sea-food orfish products.

SUMMARY OF THE INVENTION

According to the invention, there is provided a gas mixture comprisingcarbon dioxide having a partial pressure in the range of 50% to 68%,oxygen having a partial pressure in the range of 5% to 20% and argonhaving a partial pressure in the range of 27% to 45%. These gas mixturesextend the shelf life of sea-food products and fish from 15 to 21 daysat a storage temperature of about 4° C., for a bacteriologicalcontamination of said products which is not higher than 10⁴ germs/g.

These gas mixtures slow down the enzymatic reactions and/or themicrobiological development inside and/or at the surface of said fishand sea food products.

A storage temperature of 4° C. means preferably that this temperature isfrom 4° C.+1° C. to 4° C.-3° C. i.e., about between 5° C. and 1° C. Incase of salted products, this temperature may be less.

According to a preferred embodiment of the invention, the partialpressure ranges are respectively between 50% to 65% for CO₂, between 10%and 20% for oxygen and between 30% to 40% for argon (at about the sametemperature and bacteriological contamination). According to anotherpreferred embodiment of the invention, it has been unexpectedly foundthat a gas mixture comprising about 30% argon, 50% CO₂ and 20% O₂(partial pressures) is able to extend the shelf life of said sea-foodproduct or fish to a period longer that 21 days without reaching thetolerable level of contamination (10⁴ germs/g of said product or fish).

DETAILED DESCRIPTION OF THE INVENTION

Other and further features of the invention will be clearly understoodby referring to the following examples and tables which have been chosenfor purpose of illustration, along in with the claims.

EXAMPLES 1 TO 3

Fish scallops and shrimps have been selected to perform the followingcomparative examples. They have been packaged in a gas impermeablecontainer under about atmospheric pressure. A compensated vacuumtechnique a gas flushing technique, said container being furtherheat-sealed to avoid substantially any contamination from the externalatmosphere surrounding said container. The packaged products were storedat a temperature of 4° C. under atmospheres respectively of air (example1), 30% N₂, 50% CO₂ and 20% O₂ (example 2), both according to prior art,and 30% argon, 50% CO₂ and 20% O₂ (example 3), according to theinvention. Results regarding the contamination type after 7, 14 and 21days of storage are summarized in the following Table 1.

                                      TABLE 1                                     __________________________________________________________________________                                                       total                                                      total total        Germs/g                    Contamination type                                                                             Staphylococcus                                                                        pseudomonas                                                                          Mesophilic                                                                          Anaerobic                                                                           Lactobacillus                                                                        of product                 __________________________________________________________________________    0  Air           2.1     3.7    4.6   2.2   2.5                                  30% Ar, 50% CO.sub.2, 20% O.sub.2                                                           2.1     3.7    4.6   2.2   2.5                                  30% N.sub.2, 50% CO.sub.2, 20% O.sub.2                                                      2.1     3.7    4.6   2.2   2.5                                7 Air           1.9     5.9    7.1   3.0   2.9                               days                                                                             30% Ar, 50% CO.sub.2, 20% O.sub.2                                                           1.5     2.9    3.5   3.0   2.8                                  30% N.sub.2, 50% CO.sub.2, 20% O.sub.2                                                      1.4     2.9    3.0   3.5   2.0                               14 Air           4       5.9    7.0   4.3   4.5    10.sup.7                   days                                                                             30% Ar, 50% CO.sub.2, 20% O.sub.2                                                           2.2     3.0    3.7   2.8   2.5    7 10.sup.3                    30% N.sub.2, 50% CO.sub.2, 20% O.sub.2                                                      1.7     3.9    4.3   3.8   3.3    3.6 10.sup.4               21 Air           3       5.7    6.8   6.4   6      10.sup.7                   days                                                                             30% Ar, 50% CO.sub.2, 20% O.sub. 2                                                          1.7     3.2    3.7   3.3   2.8    10.sup.4                      30% N.sub.2, 50% CO.sub.2, 20% O.sub.2                                                      2.5     3.9    5.0   3.9   3.7    10.sup.5                   __________________________________________________________________________     Results are expressed in decimal logarithmic values/g of product storage      temperature: 4° C.                                                

From these results, it can be seen that after 21 days of storage, the30% Ar, 50% CO₂, 20% O₂ mixture has a total level of bacteriologicalcontamination close to 10⁴ germs/g of product as compared to more than10⁵ germs/g of product for the 30% N₂, 50% CO₂, 20% O₂ mixture. Bearingin mind that 10⁵ germs/g is, in various countries, the tolerable limitfor the bacteriological contamination level of a fresh product, it canbe seen from these results that the gas mixture (30% Ar, 50% CO₂, 20%O₂) is able to extend the shelf-life of sea-food for a period longerthan 21 days without reaching the tolerable level of contamination.

EXAMPLES 4 TO 6

The chemical and enzymatical degradation of the products stored (asexplained before) under air and modified atmospheres were examined aswell. The amount of volatile nitrogen (Amino compounds produced fromchemical and enzymatical degradation of proteins) was measured in everysample with the mean of the total volatile amino bases method (TVAB),for fresh scallops and shrimps. The results obtained are summarized inTable 2.

                  TABLE 2                                                         ______________________________________                                        Product                                                                       Scallops            Shrimps                                                                 30% N.sub.2                                                                            30% Ar      30% N.sub.2                                                                          30% Ar                                            20% O.sub.2                                                                            20% O.sub.2 20% O.sub.2                                                                          20% O.sub.2                         storage                                                                              Air    50% CO.sub.2                                                                           50% CO.sub.2                                                                         Air  50% CO.sub.2                                                                         50% CO.sub. 2                       ______________________________________                                         0 days                                                                              1.31   1.31     1.31   0.54 0.54   0.54                                 7 days                                                                              5.45   3.9      4.4    4.2  0.8    0.6                                 14 days                                                                              12.9   7.3      6.4    7.8  4.2    1.6                                 21 days                                                                              14.6   13.0     7.5    13.3 12.0   4.9                                 ______________________________________                                    

From the above table 2, it is obviously shown that the gas mixtureaccording to the invention has considerably slowed the chemical andenzymatical degradations of fresh sea-food products.

The use of a mixture according to the invention furthermore provides aninhibition effect on the growth rate of some microorganisms, such asfungi like Neurospora crassa, as argon is known to have such an effect(see for example "General Biological Effects of the Helium--Xenon Seriesof Elements" H. R. SCHREINER. Federation Proceedings--Vol 27--No.3--May-June 1968--or "Growth Response of Neurospora Crassa to IncreasedPartial Pressures of the Noble Gas and Nitrogen"--R. G. Bachheit, H. R.Schreiner, G. F. Doebler--Journal of Bacteriologics, February1966--American Society for Microbiology).

We claim:
 1. A gas mixture for fish and sea-food products preservationcomprising, in partial pressure, about 50% to 68% carbon dioxide, 5% to20% oxygen and 27% to 45% argon, said mixture slowing down enzymaticreactions at the surface and inside said fish and sea-food products. 2.A gas mixture for fish and sea-food products preservation comprising, inpartial pressure, about 50% to 68% carbon dioxide, 5% to 20% oxygen and27% to 45% argon, said mixture slowing down the microbiologicaldevelopment at the surface and inside said fish and sea-food products.3. A gas mixture according to claim 1, wherein said mixture comprisesabout 50% to 65% carbon dioxide, 10% to 20% oxygen and 30% to 40% argon.4. A gas mixture according to claim 1, wherein said mixture comprisesabout 50% CO₂, 20% O₂ and 30% argon.